Call Slot In Qt
Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. To establish a signal and slot connection between two widgets in a dialog, you first need to switch to Qt Designer's Edit Signals/Slots mode. To do that, you can press the F4 key, select the EditEdit Signals/Slots option in the main menu, or click on the Edit Signals/Slots button on the toolbar.
Introduction
Remarks
Official documentation on this topic can be found here.
A Small Example
Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.
The minimal example requires a class with one signal, one slot and one connection:
counter.h
The main sets a new value. We can check how the slot is called, printing the value.
Finally, our project file:
The new Qt5 connection syntax
The conventional connect syntax that uses SIGNAL and SLOT macros works entirely at runtime, which has two drawbacks: it has some runtime overhead (resulting also in binary size overhead), and there's no compile-time correctness checking. The new syntax addresses both issues. Before checking the syntax in an example, we'd better know what happens in particular.
Let's say we are building a house and we want to connect the cables. This is exactly what connect function does. Signals and slots are the ones needing this connection. The point is if you do one connection, you need to be careful about the further overlaping connections. Whenever you connect a signal to a slot, you are trying to tell the compiler that whenever the signal was emitted, simply invoke the slot function. This is what exactly happens.
Here's a sample main.cpp:
Hint: the old syntax (SIGNAL/SLOT macros) requires that the Qt metacompiler (MOC) is run for any class that has either slots or signals. From the coding standpoint that means that such classes need to have the Q_OBJECT macro (which indicates the necessity to run MOC on this class).
The new syntax, on the other hand, still requires MOC for signals to work, but not for slots. If a class only has slots and no signals, it need not have the Q_OBJECT macro and hence may not invoke the MOC, which not only reduces the final binary size but also reduces compilation time (no MOC call and no subsequent compiler call for the generated *_moc.cpp file).
Connecting overloaded signals/slots
While being better in many regards, the new connection syntax in Qt5 has one big weakness: Connecting overloaded signals and slots. In order to let the compiler resolve the overloads we need to use static_casts to member function pointers, or (starting in Qt 5.7) qOverload and friends:
Multi window signal slot connection
A simple multiwindow example using signals and slots.
There is a MainWindow class that controls the Main Window view. A second window controlled by Website class.
The two classes are connected so that when you click a button on the Website window something happens in the MainWindow (a text label is changed).
I made a simple example that is also on GitHub:
mainwindow.h
mainwindow.cpp
website.h
website.cpp
Project composition:
Call Slot In Qts
Consider the Uis to be composed:
- Main Window: a label called 'text' and a button called 'openButton'
- Website Window: a button called 'changeButton'
So the keypoints are the connections between signals and slots and the management of windows pointers or references.
This article mainly introduces PyQt5 daily must learn events and signals related information, has some reference value, interested partners can refer to
In this section we will explore how PyQt5’s events and signals are implemented in the application.
Book: Create Desktop Apps with Python PyQt5
Events
All GUI applications are event-driven. Application events are generated primarily from users, but they can also be generated by other means, such as an Internet connection, a window manager, or a timer. When we call the exec_() method of the application, the application enters the main loop. The main loop detects various events and sends them to the event object.
In the event model, there are three participants.
- event source
- event object
- event target
An event source is a change in the state of an object that generates an event. An event object (event) is an object that encapsulates a state change in the event source. The event target is the object that wants to be notified. The event source object represents the task of processing an event to the event target.
PyQt5 uses a unique signal and slot mechanism to handle events. Signals and slots are used for communication between objects, and when a particular event occurs, the signal is fired. The slot can be any Python call. The connection to the slot is called while the signal is transmitting.
Signals & slots
Call Slot Directly Qt
Here’s a simple example to demonstrate PyQt5’s signal and slot.
In our example, QtGui.QLCDNumber and QtGui.QSlider will be used. we change the LCD numbers by dragging the slider.
Here, the slider’s valueChanged signal is connected to the LCD’s display (display) slot.
A transmitter is an object that sends a signal. The receiver is the object that receives the signal. What slots is the method of feedback to the signal.
Overwrite the system event handler. You can use any key to fire an event.
In the example below the escape key triggers an event that quits the program.
In our example, we reimplement the keyPressEvent() event handler.
If we press the Esc key on the keyboard, the application terminates.
Book: Create Desktop Apps with Python PyQt5
Event sender event send
To facilitate differentiation of multiple event sources connected to the same event target, the sender() method can be used in PyQt5.
In our example there are two buttons. Both buttons are connected to the buttonClicked() method and we respond to the clicked button by calling the sender() method.
The two buttons are connected to the same slot.
We determine the signal source by calling the sender() method. In the application’s status bar, the label of the button that was pressed is displayed.
Call Slot In Qtc
Customized emission signals
An object created from a QObject can signal. In the following example, we’ll look at how we can customize the signal sent.
We create a new signal called closeApp. This signal is emitted by the mouse press event. This signal is connected to the close() slot in QMainWindow.
Call Slot In Qt Code
Creates a Communicate class inherited from QObject, which has a property of the pyqtSignal() class.
Connect our custom closeApp signal to the close() slot in QMainWindow.
The CloseApp signal is emitted (emit) when our mouse clicks in the program window: application termination.
Book: Create Desktop Apps with Python PyQt5